In recent years, flat-screen TV sets and notebook-size personal computers have come into wide use, resulting in increasing demands for liquid crystal displays, organic EL displays, electronic paper displays, and the like. For driving devices in these displays, field-effect transistors (FETs) are used. FETs making use of silicon, an inorganic material, are primarily used these days, but reports have been made about displays that use organic transistor devices for lower manufacturing cost, larger screen size and flexibilization.
Most of such displays, however, rely upon a combination of organic field-effect transistors (OFETs) and liquid crystal or electrophoretic cells. OFET can hardly provide a large current due to its structure and low mobility. Practically no report has hence been made about a case in which OFETs are used as drive devices in an organic EL display, said drive devices being current-driven devices that require a large current. There is, accordingly, an outstanding desire for the development of organic transistor devices that can output a large current at a low voltage and can drive an organic EL display.
For obtaining a large current with OFET, its channel length needs to be shortened at present. With a patterning technology developed with a view to mass production, however, it is difficult to shorten the channel length to several micrometers or less. To resolve this problem, research is under way about “vertical-type transistor structures” that can obtain a large current in a low voltage range by causing a current to flow in the thickness direction of films. Film thicknesses generally employed in vertical-type transistors range from several tens nanometers to several hundreds nanometers, and can be controlled with high accuracy on the order of several angstroms. The fabrication of a vertical-type transistor with its channel extending in the film thickness direction (vertical direction) has a possibility that a channel length of 1 μm or less may be easily realized to obtain a large current. As such vertical-type organic transistor devices known to date, there are vertical-type transistors of a polymer-grid triode structure, in each of which a self-organizing network structure in the form of a polyaniline film is used as a grid electrode, and static induction transistors (SITs) in each of which a source-drain current is controlled by modulating the width of a depletion layer in a microstripe-shaped intermediate electrode.
Recently, there has been proposed a vertical-type organic transistor device that has a stacked structure of organic semiconductor/metal/organic semiconductor and can show high-performance transistor characteristics (PTL 1). This vertical-type transistor device is provided, between an emitter electrode and a collector electrode, with organic semiconductor layers and a stripe-shaped intermediate metal electrode. Upon passage of electrons, which have been injected from the emitter electrode, through the intermediate metal electrode in the organic transistor device, current amplification similar to that available from a bipolar transistor is observed, and the intermediate metal electrode serves like a base electrode. The vertical-type transistor device is, therefore, called a “metal-base organic transistor” (which may hereinafter be called “MBOT”).
In MBOT, no substantial current flows when an output voltage is applied between the emitter electrode and the collector electrode and no voltage is applied between the emitter electrode and the base electrode, but a current flows between the emitter electrode and the collector electrode when a voltage is applied between the emitter electrode and the base electrode. The current that flows between the emitter electrode and the collector electrode is a collector current, while the current that flows between the emitter electrode and the base electrode is a base current. Compared with the base current that increases upon application of a base voltage, the collector current rapidly increases. MBOT, therefore, can serve as a device that can modulate the collector current by the base voltage. A “leakage current” that may happen to flow when a voltage is applied between the emitter electrode and the collector electrode but no voltage is applied between the emitter electrode and the base electrode is an off-state current, while a current that flows upon application of a voltage between the emitter electrode and the base electrode is an on-state current. MBOT is a transistor device that allows substantially no off-state current and can provide a large on-state current.
As the structure of an organic transistor (MBOT), there has been reported MBOT that can be readily fabricated by providing a transparent ITO electrode as a collector electrode and stacking organic semiconductor/metal/organic semiconductor on the transparent ITO electrode by vacuum deposition (PTL 2). Employed as the organic semiconductors are N,N′-dimethylperylenetetracarboxylic acid diimide (Me-PTCDI) and fullerene (C60), which are n-type organic semiconductor materials. Employed as electrode materials, on the other hand, are Al for a base electrode and Ag for an emitter electrode. By introducing a dark-current suppression layer and subjecting the base electrode to heat treatment, this MBOT serves as a transistor device that can perform large-current amplification with an improved on/off ratio (ratio of on-state current to off-state current). As is appreciated from the foregoing, MBOT is characterized in that it does not need micropatterning for a microgrid-shaped electrode or microstripe-shaped electrode although it is a vertical-type transistor.
Also reported are organic transistor devices (MBOTs), each of which is reported to provide good current amplification characteristics and on/off ratio without applying heat treatment or the like. They are MBOTs each having organic semiconductor layers and a sheet-shaped base electrode between an emitter electrode and a collector electrode and also having an energy barrier layer and a charge-pass promoting layer between the base electrode and the collector electrode (PTL 3); and MBOT making uses, as a collector layer, of an organic semiconductor layer formed from a perylenetetracarboxylic acid diimide having long-chain alkyl groups (Japanese Patent Application 2009-114619).
As a vertical-type transistor, an organic transistor provided with a light-transmitting metal substrate has also been reported as a bipolar transistor. This organic transistor has organic semiconductor layers and a sheet-shaped base electrode between an emitter electrode and a collector electrode, and as the organic semiconductor layers, one being between the emitter electrode and the base electrode and the other between the collector electrode and the base electrode, uses hetero-junction organic semiconductor layers formed from N,N′-diphenyl-N,N′-di(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPD) and fullerene (C60), respectively (NPL 1).